The invention concerns a device for the melting or refining of glasses or glass ceramics.
Such devices have become known in the configuration of the so-called xe2x80x9cskull potxe2x80x9d. They comprise a pot walling. This is generally cylindrical. It is constructed of a crown of vertical metal pipes. Slots remain between adjacent pipes. The bottom of the pot can also be constructed of metal pipes. However, it can also consist of refractory material. The ends are connected to vertical pipes for the introduction of cooling agent or for the discharge of cooling agent.
Heating is conducted by means of an induction coil, which surrounds the pot walling and by means of which high-frequency energy can be input into the contents of the pot.
Such a skull pot has been made known, for example, from EP 0 528,025 B1.
A skull pot operates as follows: The pot is filled with a [fresh] glass batch or refuse glass or a mixture thereof. The glass or the melt must first be preheated in order to obtain a certain minimum conductivity. Preheating is primarily conducted by means of burner heating. If the temperature for [HF energy] input has been reached, then further energy input can be supplied by means of irradiation by high-frequency energy. During the operation, in addition to the high-frequency energy heating, the melt is also heated by means of burners, which operate from the top onto the melt, or by means of hot off-gases. This additional heating is particularly necessary in the case of the use of a skull pot for refining. That is, if the surface layer is cold and correspondingly highly viscous, then bubbles will be prevented from exiting the melt or a foaming will occur.
Usually, the skull pot is arranged in a standing position. It is generally operated discontinuously.
JP 57 [1982]-95,834 describes a device with a quartz channel, which is arranged horizontally.
A high-frequency oscillating circuit, which contains a cylindrical coil, is assigned a to the quartz channel. The cylindrical coil wraps around the quartz channel. The quartz channel is actually cooled, However, when aggressive glasses are melted, it does not have a high long-term stability and a high breaking strength. In addition, a special heating of the melt surface is not possible. In fact, a certain cooling occurs, which can lead to the formation of a tough skin in the surface region. If such a channel is to be used as a refining device, then bubbles can no longer rise up unhindered and be discharged from the melt. The channel therefore cannot be used for refining. If the channel is used for melting, and the melt contains readily volatile components, then there is a risk of condensation at the cooled superstructure of the channel. The condensate can thus drip into the melt in an uncontrolled manner. This can lead to glass defects in the form of nodes, blisters or streaks. If corrosion of the coil material occurs, then this leads to discoloration of the glass, depending on the material of the coil. This is not acceptable, particularly in the case of optical glasses.
Further, there are very many optical glasses, which have a high proportion of fluorine, phosphate or other highly aggressive components. These can also attack the material of the coil. The corrosion can be strong enough that discharge of cooling water occurs, so that the operational safety of the plant is no longer assured.
The object of the invention is to create a device, in which the advantages of the technique of inductive heating are utilized, which is reliable in operation, which is also suitable for the refining of melts, and which leads to glasses of a perfect quality. This object is resolved by the features of claim 1.
According to the invention, not only is use made of the high-frequency technique, but also the skull technique is used. A channel is used, which has a structure similar to that of a skull pot. The upper space is not covered by water-cooled pipes in this way. Rather, it is freely accessible and can be used for thermal insulation or for an additional heating by means of a burner or by means of radiant heat.
The invention, however, introduces the following additional advantage, which the inventors have recognized:
If the water-cooled metal pipes of a skull device run in the direction of the glass flux, then flashovers between the glass melt and the metal pipes of the skull channel can occur at high melt temperatures, if the solidified cold glass insulation layer is very thin. This can lead to arcing between the skull channel and the melt, which can have as a consequence a disruption of the skull frame. It is presumed that the arc formation is produced by high-frequency voltages induced in the skull pipe.
In one embodiment according to the invention, the water-cooled metal skull pipes run perpendicular to the direction of glass flow, thus not in the direction of glass flow. In this way, the formation of arcs between the skull pipes and the melt is extensively avoided.
In another embodiment of the invention, the tendency toward flashover, i.e.: the tendency to form arcs, is fully prevented in that the ends of the U-shaped piece of the skull pipe are joined with each other in a conductive manner for purposes of forming a short-circuit bridge [shunt].
The invention introduces the following additional advantages:
It is excellently suitable for continuous operation. It can thus operate very economically.
Another advantage consists of the following:
Due to the configuration and arrangement of the induction coils in the lying-down position, the channel is open at the top. The level of the melt is exposed. The surface of the melt is thus freely accessible for the installation of an additional heating device, for example, a gas burner or an electrical heating device. This top heating is of particular advantage for the case when the channel is utilized as a refining aggregate. High surface temperatures can be obtained accordingly, so that the bursting of bubbles is assured in the region of the surface.
The heating from above is also helpful if high-frequency energy failure occurs. In this way, at least the glass transport can be assured. Also, the melt temperature can be maintained at such a value that a recoupling is possible when high-frequency heating is again started up.
Further, there is no danger of condensation of products of evaporation on the water-cooled coil pipes, since these are not found above the level of the melt.
Additionally, a complex superstructure is provided in the case of the skull channel according to the invention, which includes ceramic plates that cover thechannel. The ceramic plates can be heated on the top side by means of burners. The plates then radiate heat onto the glass surface by their lower side, so that the glass is indirectly heated. This has the advantage that strong and turbulent atmospheric interferences do not occur directly below* the level of the glass melt in the case of glasses containing components that have a high tendency toward evaporation (B2O3, P2O5, F, S, Se, Te or the like). Such interference would entrain the easily volatile components, which would lead to a modification of the glass composition. A premature blockage of filter devices is also avoided in this way.
* sic; above?xe2x80x94Trans. note. 
Another advantage of the skull channel according to the invention lies in the fact that when additional heating is produced by means of burners, with or without ceramic cover, a reducing atmosphere can be established. This is necessary for the production of thermal insulation glasses or glasses with high UV transmissivity, in which it happens that the Fe3+/Fe2+ ratio is shifted as extensively as possible to the reduced form. Fe2+, which absorbs in the IR, and thus is used for heat radiation (thermal insulation glass), whereas Fe3+, which absorbs in the UV, must be avoided as extensively as possible in the case of glasses with high UV transmissivity. Since these glasses are often phosphate or fluorophosphate glasses, the use of a ceramic cover plate is important. A similar argument applies to the production of initial glasses, in which it happens that the chalcogenides necessary for coloring are present at least partially in reduced form (S2xe2x88x92, Se2xe2x88x92, Te2xe2x88x92). Here, it is also of advantage to minimize evaporation, in this case of color components, by the use of ceramic cover plates.
Reducing conditions may also be established with the use of electrical heating from above by means of corresponding reducing gases or gas mixtures (forming gas, H2, CO/CO2 and others), but the use of an adjusted burner to produce a reducing atmosphere (incomplete gas combustion, i.e., a smaller quantity of air/oxygen) is generally more cost-favorable.
The described channel systems may be joined by flanges to conventionally heated platinum or stone channels. When connected to a stone channel, the cooling of the stone channel-skull transition region is important. In operation, usually a good contacting of the water-cooled channel with the stone material is sufficient. During the heat-up phase, the freedom of motion of the stone channel must be assured relative to the HF channel, since the stone channel extends during tempering, whereas the water-cooled HF channel retains its geometry. The procedure of moving the stone channel up to the HF channel only after tempering and attaching it in the hot state has proven optimal.
When an HF channel is contacted with an electrically heated platinum channel, it must be assured that either there is no electrical contact between the metal components of the HF channel or, however, there is a very good electrical contact. The latter case conceals the danger that HF interference signals might be decoupled by means of the platinum system, but is preferred to the poor contact, which is accompanied by spark formation at places with increased resistance.
A complete electrical separation between skull channel and platinum channel can be achieved by ceramic intermediate pieces, which must assure a distance of at least 5 mm between metal components. Greater distances offer more security relative to electric breakdown strength, but are more difficult to seal, particularly in the case of aggressive melts. A quartz ceramic has proven most suitable as insulation material.
If the channel has a length of more than 1200 mm, then it must be heated with several flat coils, whereby the flat coils are ideally provided with energy by different HF generators, in order to be able to adjust the temperature in the individual channel regions, independently of one another. The distance x between adjacent flat coils should be greater than or at least equal to the height of the coil winding d, and thus the HF fields cannot mutually influence one another.
An unheated or only very weakly heated region lies in the transition region between two flat coils, since the two flat coils cannot be randomly guided next to one another. The melt cools down in this zone. An up-and-down heating of a glass melt is undesired for glass quality, and particularly also due to the danger of thermal reboil. In order to assure a flat temperature profile or a monotonically rising or monotonically falling temperature profile over the entire channel length, an additional heating device must be installed in the transition region between two coils. In the channel type described here, either an additional electrical heating device (e.g., sic rods or kanthal needles) or a gas firing can be utilized. In the case of gas firing, the use of flat coils and guiding the coil just below the channel has proven advantageous.